One should also bear in mind that copyright law, like any law, only
takes effect when someone (the copyright owner) cares to take legal
action because they feel their rights and entitlements are being
infringed to their detriment (eg. loss of income), the legal action
goes to a court of law, and a prosecution is successful, and the
punishment made.

The current copyright owners of RDH4 may or may not know that pirate
PDF scans or CD-Rs exist on Internet, or if they do know, they may not
care. If people want to buy a legitimate hard cover volume (which is
in many ways is better reading dodgy scans via Adobe Reader), they
will actively seek out a book shop on online book seller who has it.

I have 3 different hard cover originals of RDH4, as well as hard
covers of RHD3, 2 and 1. I'm happy.

Glenn.